Problem: Which of these numbers is prime? ${9,\ 12,\ 25,\ 41,\ 51}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 12 are 1, 2, 3, 4, 6, and 12. The factors of 25 are 1, 5, and 25. The factors of 41 are 1 and 41. The factors of 51 are 1, 3, 17, and 51. Thus, 41 is a prime number.